\centering
\begin{tabular}[t]{c>{\centering\arraybackslash}p{15mm}rrrrcccc}
\toprule
\multicolumn{2}{c}{ } & \multicolumn{4}{c}{Log likelihood} & \multicolumn{4}{c}{Perplexity} \\
\cmidrule(l{3pt}r{3pt}){3-6} \cmidrule(l{3pt}r{3pt}){7-10}
\multicolumn{2}{c}{ } & \multicolumn{2}{c}{ } & \multicolumn{2}{c}{2-D} & \multicolumn{2}{c}{ } & \multicolumn{2}{c}{2-D} \\
\cmidrule(l{3pt}r{3pt}){5-6} \cmidrule(l{3pt}r{3pt}){9-10}
Survey & Avg. no. of items & Null & 1-D Mix. & No mix. & Mix. & Null & 1-D Mix. & No mix. & Mix.\\
\midrule
2012 & 18.6 & -661043 & -550889 & -551832 & -550929 & 1.93 & 1.74 & 1.75 & 1.74\\
2013 & 21.8 & -226870 & -194338 & -193809 & -193867 & 1.89 & 1.74 & 1.74 & 1.74\\
2014 & 31.6 & -1156518 & -960685 & -956933 & -949695 & 1.92 & 1.74 & 1.73 & 1.72\\
2015 & 32.5 & -292295 & -230744 & -227953 & -226884 & 1.89 & 1.66 & 1.66 & 1.65\\
2016 & 28.8 & -1137878 & -972217 & -973954 & -971166 & 1.86 & 1.71 & 1.71 & 1.71\\
\addlinespace
2017 & 30.9 & -358915 & -269839 & -269580 & -267369 & 1.90 & 1.64 & 1.64 & 1.63\\
2018 & 33.1 & -1274454 & -973250 & -975855 & -968311 & 1.91 & 1.66 & 1.67 & 1.66\\
\bottomrule
\end{tabular}
